1. Field of the Invention
This invention relates to the enhanced detection of non-voice information on a telephone line. More particularly, it relates to the suppression of voice signals at off-hook customer premises equipment to allow detection of call information regarding an incoming third party call while in an off-hook condition, e.g., the alerting CAS tone, FSK and other Caller ID information.
2. Background of Related Art
Call information is useful to users of a telephone system. For example, a well known and popular call information service provided in the United States is Calling Identity Delivery (Caller ID). This service typically provides the telephone number and household name information of a calling party to the called party before the call is answered. Based on a display of the call information, the called party may decide not to answer the incoming call. Basic call information such as Caller ID information is transmitted from the local telephone company to the called party while the called party's phone is in a hung-up or on-hook state, e.g., between the first and second rings.
Another telephone company service which has become well known and popular is that which allows a third party to call while the telephone is off-hook, currently implemented in the United States in Call Waiting services provided by telephone companies. Call waiting allows someone who is already using the telephone (i.e., in an off-hook state), to receive an audible interruption, click or other indication at the customer premises equipment that another person is calling, and then to establish a connection with the third party caller without hanging up on the first party.
More recently, call information has been combined with third party caller services to provide an advanced service currently known as Calling Identity Delivery on Call Waiting (CIDCW) in the United States. CIDCW service allows a customer, while off-hook on an existing call, to receive information about an incoming calling party on a waited call before answering that incoming call waiting call. Transmission of the third party's call information with this service takes place almost immediately after the customer premises equipment (CPE) is alerted to the new call with a CPE Alerting Signal (CAS) which is a short burst of a combination of high frequencies that are appended to the Subscriber Alerting Signal (SAS) otherwise known as the `Call Waiting Tone`. Using CIDCW, a user can decide whether or not to take the incoming call waiting call.
Customer premises equipment capable of receiving on-hook call information such as Caller ID (CID) including Calling Number Delivery (CND) and Calling Name Delivery (CNAM), is generally referred to as Type 1 customer premises equipment. Type 1 customer premises equipment additionally capable of receiving call information when off-hook or already in an established call with another party (CIDCW) is generally referred to as a Type 2 customer premises equipment. With either Type 1 or Type 2 customer premises equipment and Caller ID service, the called party is alerted to the availability of call information for an incoming call waiting call with an alerting CAS tone. The alerting CAS tone is acknowledged with a DTMFD, and the FSK data corresponding to the call information follows from the central office to the customer premises equipment.
In the United States, FSK call information such as Caller ID generally is not transmitted in conventional schemes by the central office unless an acknowledge (ACK) signal is received from the customer premises equipment. Of course, the call information may be transmitted without implementation of an acknowledge signal. In any event, in order to prevent the customer premises equipment from hearing the audible ACK and FSK call information signals, the central office silences the far-end party's voice path before providing the alerting CAS sequence. Likewise, the customer premises equipment mutes the handset (or microphone and speaker of a speakerphone) before sending the ACK signal. The customer premises equipment sends FSK caller ID call information using the signaling protocol described in Bellcore recommendation TR-NWT-000030. Upon completion of the transmission of the FSK call information, or if no ACK signal is received by the central office from the customer premises equipment, the central office restores the far-end party's talking path. The customer premises equipment restores handset (or microphone and speaker) operation upon completion of the reception of the FSK call information.
At any time after a called party having CIDCW service has been alerted to the availability of call information for an incoming call waiting call and while a party is still in the waited state, CIDCW allows the customer to flash the switch hook to retrieve the waited call, and subsequently to go back and forth between the current far-end party and the held party by flashing. CIDCW provides all the capabilities associated with the current CW service, with the additional capability of providing CID data to a customer on waited calls. Therefore, CIDCW is considered an enhancement of the CW service. A telephone line may either have call waiting (CW) or CIDCW service enabled, but not both at the same time.
In the on-hook condition, there is little concern about a user hearing the alerting CAS tones or subsequent FSK data because the phone is not being used when the call information is being transmitted. Moreover, there is no concern over frequencies of a conversation interfering with their detection because there is no conversation transmitted when the telephone is on-hook. Thus, the detection of the call information in Type 1 customer premises equipment is relatively straight forward and can be performed simply by a detection algorithm in a digital signal processor (DSP) at the customer premises equipment.
The more difficult situation arises in Type 2 customer premises equipment because the telephone is off-hook, i.e., is carrying a conversation or other information between at least two parties. Thus, it is very possible for voice signals of the conversation to clash with and be disrupted by the alerting CAS tones indicating the availability of call information for an incoming call waiting caller. This causes an annoyance to the user at the customer premises equipment of hearing the alerting CAS tones in the earpiece or speakerphone. Most of this annoyance is conventionally avoided by switching out the earpiece or speakerphone for the remaining portion of the alerting CAS tones and FSK data upon detection by the customer premises equipment. While this causes an interruption in the conversation between the original two parties, it is rather brief and the conversation resumes after the call information is received by the customer premises equipment.
However, the more serious concern is the ability of the Type 2 customer premises equipment to accurately and reliably detect the alerting CAS tones while in an off-hook condition. CIDCW requires the detection of the alerting CAS tones in the possible presence of near-end speech at the customer premises equipment because the customer premises equipment does not know a priori when the alerting CAS tones are coming. FIG. 3 shows the approximate long-term average spectral energy density for continuous speech, showing that high relative spectral energy is present in near-end speech, which is in the same general range as the alerting CAS tones used to transmit call information such as Caller ID. The presence of speech adds complexity to algorithms employed to detect the alerting CAS tones and, in fact, can cause a false detect or non-detect of an alerting CAS tone signal. Thus, conversation can interfere significantly with the alerting CAS tones at the customer premises equipment, causing erroneous detection of the alerting CAS tones, and erroneous reception of the call information such as a telephone number and household name of the incoming call waiting party.
CIDCW data is received when a telephone is off-hook (i.e., in use). To guarantee that the frequency shift keying (FSK) data transmitted after the alerting CAS tones by the central office are not corrupted by conversation, Bellcore recommendation FSD 01-02-1090 suggests muting of the microphone at the customer premises equipment upon detection of the alerting CAS tone sequence, and maintaining the muting until the call information for the call waiting caller is received. Conventional CIDCW customer premises equipment follows the Bellcore recommendation FSD 01-02-1090 (which, in its entirety, is explicitly incorporated herein by reference). For instance, U.S. Pat. No. 5,263,084 and other conventional systems mute the microphone at the customer premises equipment by switching the microphone out of electrical connection with the customer premises equipment upon detection of the alerting sequence of CAS tones.
FIG. 4 shows a conventional customer premises speakerphone 500. In FIG. 4, a telephone line from a central office 518 interfaces with the telephone line interface (TLI) 514 (otherwise known as a `hybrid`) of the customer premises equipment. The TLI 514 shows the required impedance and isolation to the telephone line, e.g., typically about 50 ohms DC impedance and about 600 ohms AC impedance, according to current standards in the United States. Of course, the invention is equally applicable to customer premises equipment operating on a telephone line conforming to standards of other countries.
In this conventional speakerphone, both the speaker 564 and the microphone 562 are electrically switched out upon detection of the alerting CAS tones. Switches 454a and 454b switch the microphone 562 and speaker 564, respectively, out of electrical connection to prevent conversation or voice from corrupting the signal analyzed by the alerting CAS tone functional block 510 while the subsequent call information such as alerting CAS tones and Caller ID FSK data are transmitted.
The DSP 511 includes a CAS tone detection and FSK call information algorithm (CAS tone functional block) 510 and other functionality (not shown) for providing the functions of a conventional speakerphone. A tone detector 510b and a frequency shift keying (FSK) demodulator 510a, respectively detect the alerting CAS tones and subsequent FSK data containing the call information. A tone generator 510c provides return information to the central office 518.
In operation, DSP 511 receives a signal from the telephone line via the hybrid or TLI 514 and a codec 512. An A/D and D/A converter within the DSP 511 can be used to replace the codec 512 in cost sensitive applications. An audio echo canceler (AEC) algorithm 592 in the DSP 511 helps avoid the possibility of feedback of audio from the speaker 564 reflecting off walls in the room and back into the microphone 562 causing undesirable squealing and other uncomfortable noise. The AEC algorithm 592 suppresses audio picked up by the microphone 562 which was output from the speaker 564, eliminating or minimizing acoustic feedback. The AEC algorithm 592 is an adaptive, speech trained acoustic echo canceler, and is under the control of a host processor. The DSP 511 may double as a host processor, or a separate microcontroller, microprocessor, or other processor may serve as a host processor for the customer premises equipment. The amount of acoustic echo cancellation provided by the AEC algorithm 592 is monitored by the DSP 511 and adjusted as necessary for optimal performance of the speakerphone.
The audio signal is passed to the speaker 564 from the DSP 511 through codec 560. Similarly, the audio signal from the microphone 562 is passed to the DSP 511 through the codec 560. The switches 454a and 454b are controlled by the DSP 511 or other host processor to electrically disconnect the microphone 562 from the DSP 511 upon the detection of call information such as an alerting CIDCW CAS tone, and for the duration of the reception of subsequent FSK call information, e.g., call waiting caller ID information.
Thus, conventional systems require the microphone of a handset or a speakerphone to be disconnected, switched out of electrical connection with the DSP, or otherwise muted to allow accurate detection of the CIDCW FSK data. This switching out of the microphone prevents the optimization of the detection of the alerting CAS tones until the alerting CAS tones are first detected. The microphone cannot be muted until after the alerting CAS tone is detected. Thus, the alerting CAS tone is necessarily analyzed with underlying voice data included. Accordingly, in the face of high energy, near-end voice, the detection of the alerting CAS tones must be detected in the face of possible voice conversation frequencies. Therefore, conventional Type 2 customer premises equipment undesirably requires additional switching mechanisms which complicate the hardware and software of the customer premises equipment, require additional operational steps in receiving call information such as Caller ID information for a waiting caller, require more complicated host processor software and hardware, and otherwise decrease the reliability of the customer premises equipment.